In a disk recording and/or reproduction system an objective of the servo system which controls the radial position of the transducer on the disk, is to keep the transducer over the center of a preselected track. In a disk file system, this is done as the transducer reads position information from a track in the surface of the disk as the disk rotates. For an embedded or sectored servo system the position information is obtained from inter sector information placed at predetermined locations on the tracks of the disk. This position information is then used to develop a position error signal. The error signal is then fed back through a compensator into the drive motor for the transducer actuator to move the transducer in a direction to reduce the error.
The servo system includes a feed back servo loop (head actuator loop). The position error signal is coupled into the head actuator loop. Because of the finite response time of the feed back servo loop in correcting for disturbances, these disturbances or displacements cannot be totally eliminated. To desensitize rotary actuator disk drives to translational disturbances, a balanced mechanical actuator has traditionally been used. However, since the actuator must pivot freely in order to access the data (as free as friction will allow) the effects of rotary disturbances about the axis normal to the disk surface can be considerable.
It has been proposed to use rotational accelerometers to sense rotational shock and vibration. The rotational accelerometers generate a signal which can be used as a feed forward controller to make the disk drive more robust to shocks and vibrations. There have been numerous publications on accelerometer feed forward algorithms, but the current use of accelerometers is limited to the role of a threshold detector for stopping writes of data to the disk. Such as, Hewlett Packard, HP Kittyhawk Personal Storage Modules Product Brief, 1993, and U.S. Pat. No. 5,235,472. More specifically, the accelerometer detects a threshold level of shock or vibration and prevents writes to the disk if the shock or vibration is greater than a predetermined amount.
The idea of using accelerometer signals to compensate for external shock and vibration of a disk drive is not new. As far back as 1977, White (U.S. Pat. No. 4,040,130) proposed a scheme to use accelerometers to minimize the possibility of the magnetic heads slapping against the magnetic media. Improved mechanics and stiffer air bearings have minimized the need for such a system. White proposed two modes of using accelerometers in disk drives. The first mode suggests using the accelerometer as a simple shock protection device. When the accelerometer detects a large enough shock, the magnetic heads are moved away from the disk to avoid possible head crashes. The second mode suggest using the accelerometer in a control loop. The effect of the shock on the head to disk spacing being actively minimized by feeding the accelerometer signal into a control loop. The vertical position of the head can be controlled by either changing the internal pressure of the drive and thus the air bearing stiffness, or by using a servomotor on the drive arm in the vertical direction.
More recently, the use of accelerometers for minimizing the effects of both seek reaction torque and external excitation has been studied by Davies and Sidman (D. B. Davies and M. D. Sidman, "Active compensation of shock, vibration, and wind-up disk drives," Advances in Information Storage Systems, Vol. 5, pp. 5-20, ASME Press, 1993). Their conclusions suggest analytically calculating a filter for filtering the accelerometer response such that the effect of both of the disturbances is zeroed. With some practical constraints, they derived a workable solution. However, their solution requires knowledge of parameters characteristic of the particular drive and accelerometer being used. This solution does not discuss accelerometer resonances in the servo bandwidth or noise. This would imply that they are using expensive high grade accelerometers. They use low pass filtering to limit the gain of the accelerometer loop at high frequencies to prevent unmodeled head disk assembly (HDA) dynamics from destabilizing the system. This can include HDA characteristics which were either unknown or ignored during the design of the system.
One of the main practical issues in disk drives is a continual push towards lowering the manufacturing cost. It is not practical to use expensive laboratory grade accelerometers in the manufacture of low cost disk drives. In the work of Knowles and Hanks ("Shock and vibration disturbance compensation system for disk drives," European Patent Application 871065555.3), a linear accelerometer was used to minimize the effect of translational shock on the position error signal. The accelerometer was mounted directly on the HDA so that both internally and externally produced disturbances could be sensed. However, each of the accelerometers had to be calibrated in the drive during manufacturing, and as a result, the drive costs more to manufacture. More recently, the work of Hanks (U.S. Pat. No. 5,299,075) has shown how to calibrate accelerometers while they are still in operation. This allows less expensive accelerometers to be used and reduces the manufacturing times.
Small disk drives face several problems that have yet to become major issues for large disk drives. Small disk drives are inherently designed for portable applications. In a mobile environment, the disk drive must tolerate much more severe shock and vibration than is experienced in the traditional disk drive environment.
Small disk drives have less surface area available for storing information. To preserve disk surface area for storing data, the inter sector position information locations (also known as servo bursts) are reduced in number. Therefore, the number of inter sector locations on a given track of the disk is reduced. This provides more disk space for storing information, but also reduces the rate at which the head actuator loop receives head position correction information. Generally, the result is that the sampling rate of the head actuator loop is reduced. The reduction in sampling rate often limits the close loop bandwidth, which in turn can adversely effect the drives disturbance refection ability. This makes the head actuator loop susceptible to mechanical shock and vibration.
Portable electronic devices are becoming more popular. Many of these portable electronic devices require small disk drives. With the increasing demand for smaller disk drive systems which operate at lower servo loop sampling rates, there exists a need for higher performance shock and vibration correction systems.